From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933581AbZFOT1h (ORCPT ); Mon, 15 Jun 2009 15:27:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752920AbZFOT1a (ORCPT ); Mon, 15 Jun 2009 15:27:30 -0400 Received: from tomts13-srv.bellnexxia.net ([209.226.175.34]:42546 "EHLO tomts13-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753110AbZFOT13 (ORCPT ); Mon, 15 Jun 2009 15:27:29 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AokFAKE6NkpMQWQl/2dsb2JhbACBT9UbhA0F Date: Mon, 15 Jun 2009 15:27:20 -0400 From: Mathieu Desnoyers To: "H. Peter Anvin" Cc: Peter Zijlstra , Linus Torvalds , Ingo Molnar , mingo@redhat.com, paulus@samba.org, acme@redhat.com, linux-kernel@vger.kernel.org, penberg@cs.helsinki.fi, vegard.nossum@gmail.com, efault@gmx.de, jeremy@goop.org, npiggin@suse.de, tglx@linutronix.de, linux-tip-commits@vger.kernel.org Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods Message-ID: <20090615192720.GA9056@Krystal> References: <20090615171845.GA7664@elte.hu> <4A369508.2090707@zytor.com> <20090615184858.GD6520@Krystal> <1245091917.6741.185.camel@laptop> <20090615185907.GF6520@Krystal> <1245092561.6741.205.camel@laptop> <4A369CD8.3090505@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <4A369CD8.3090505@zytor.com> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 15:24:07 up 107 days, 15:50, 3 users, load average: 0.65, 0.48, 0.46 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * H. Peter Anvin (hpa@zytor.com) wrote: > Peter Zijlstra wrote: > > On Mon, 2009-06-15 at 14:59 -0400, Mathieu Desnoyers wrote: > >> * Peter Zijlstra (a.p.zijlstra@chello.nl) wrote: > >>> On Mon, 2009-06-15 at 14:48 -0400, Mathieu Desnoyers wrote: > >>>> we should not care that much about the performance hit of > >>>> saving/restoring the cr2 register at each nmi entry/exit. > >>> But we do, perf counters very much cares about nmi performance. > >>> > >> To a point where it cannot afford a simple register save/restore ? > >> > >> There is "caring" and "_caring_". I am tempted to ask what NMI handler > >> execution frequency you have in mind here to figure out if we are not > >> trying to optimize sub-nanoseconds per minutes. ;) > > > > Ah, well, I have no idea who expensive cr2 is, if its like a regular > > register then it should be fine. If however its tons more expensive then > > we should really avoid it. > > > > As to the freq, 100kHz would be nice ;-) > > > > Writing control registers is serializing, so it's a lot more expensive > than writing a normal register; my *guess* is that it will be on the > order of 100-200 cycles. > > That is not based on any actual information. > Then how about just writing to the cr2 register *if* it has changed while the NMI handler was running ? if (unlikely(read_cr2() != saved_cr2))) write_cr2(saved_cr2) Mathieu > -hpa > -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68